Pebble heater process and apparatus



June 3,1958 A E I 2,837,586

I PEBBLE HEATERY PROCESS AND APPARATUS Filed une'25, 1954 2 Sheets-Sheet 1 FIG. 2.

INVENTOR.

H. J. HEPP ATTORNEYS June 3, 1958 J, HEPP 2,837,586

PEBBLE HEATER PROCESS AND APPARATUS Filed aune 25, 1954 2 Sheets-Sheet 2 (I) 5 0 DC N 8 Q I u 0 Q- [L] I n 6 N 5 Q 2 P o o 8 O O o xvw 9 0 "5 Q5 I INVENTOR.

H. J. HEPP ayw w ATTORNEYS 2,837,586 PEBBLE HEATER PROCESS AND APPARATUS Harold J. Hepp, Bartlesville,

Okla, assignor to Phillips Petroleum Company,

This invention relates to an improved process for effecting chemical reactions in vapor phase in pebble heater type apparatus and to improved apparatus for effecting the process. A specific aspect of the invention pertains to an improved process for effecting the conversion of hydrocarbons in a pebble heater type reactor.

This application contains common subject matter with my copending application Serial No. 115,202, filed September12, 1949, which has matured to Patent No. 2,694,096.

Pebble heater apparatus is finding increasing favor in the conversion of hydrocarbons at elevated temperatures in the range of 1500 to about 2500 F. because of the fast heating rate, continuous operation, and other process advantages provided by the pebble heater. A conventional pebble heater utilizes a pebble heating chamber having means for passing hot combustion gas upwardly through a gravitating mass of refractory pebbles which are heated therein before gravitating through an axially positioned throat to a reaction chamber or" similar shape and construction to the pebble heating chamber. The gravitating pebbles are contacted in the conversion zone or chamber with a countercurrent stream of the hydro carbon to be reacted, or converted, so as to heat the same to reaction temperature and supply the heat of reaction required to effect the conversion. The pebbles then gravitate from the lower end of the reactor to the bottom of an elevator which lifts them to a point above the top of the pebble heater chamber from whence they gravitate to that chamber for reheating and gravitation to the reaction chamber. The system from the pebble inlet in the heating chamber to a pebble feeder device in the conduit between the bottom of the reaction chamber and the lower end of the elevator is maintained full of a compact gravitating mass of pebbles at all times during operation of the unit.

Up to the present time there is no known way for maintaining the top surface of a pebble bed flat and horizontal sov that the heating effected in the upper section of the pebble bed in the reaction chamber is uniform over the entire cross section of the bed. As a result of this non-uniform heat exchange the reactant gas is not uniformly converted or reacted to the same degree in all sections of the pebble bed where the reaction is taking place. In the cracking of hydrocarbons in a pebble heater reactor the lack of uniformity in heating in the section of the bed where cracking is taking place, i. e., the top Section, the depth of cracking is not the same for all points in the bed over a transverse cross section of the bed within the reaction area. This non-uniformity in conversion, such as cracking of hydrocarbon material, is well known in the art and various attempts have been made and methods devised to decrease the non-uniformity of conversion and approach uniform conversion or cracking. Lack of uniformity in conversion or conversion conditions is at least in part the result of the manner in which hot pebbles are introduced to the reactor. In most instances the pebbles are introduced to the upper section of the reactor at a point below the dome thereof through Patented June 3, 1958 FIC6 an axial conduit and the pebbles spread out in a generally conical configuration to the juncture of the pebble bed with the periphery or the wall of the reactor. Pebble flow in this conical top section of the pebble bed is not uniform inasmuch as some of the pebbles travel laterally as well as vertically, while others travel only vertically toward the pebble outlet in the bed'of the reactor. Hence the pebbles traveling a greater lateral distance through the reactor have a greater residence time in the reaction section of the bed and are contacted with greater volumes of reactant gas than are pebbles which travel more directly to the pebble outlet in the bottom of the reactor. in other types of pebble heater apparatus the pebbles are introduced to the reactor in a plurality of streams through a plurality of conduits distributed around the dome of the reactor in a generally symmetrical pattern intermediate the axis and the periphery of the reactor.

The heated pebbles entering the reactor through the axial duct are disposed in such a manner that the top surface of the pebble bed is conical in shape in accordance with the angle of repose of the pebbles. The conical-top bed results in poor distribution of the heated pebbles in the bed and a non-uniform flow pattern of the reactant gases through the bed in that the axial element of the bed is usually hotter than an outer element and the linear velocity of the reactant gases through the bed is greater adjacent the walls of the reactor than in the center of the bed. When converting reactant material in the pebble heater reactor, such as cracking of a hydrocarbon stream, the non-uniform temperature profile in the top of the pebble bed, where a substantial portion of the conversion takes place, results in an over-conversion of a part of the feed and an under-conversion of another part. Also, the non-uniform flow of reactant gases through the bed results in under-conversion in the element of the bed adjacent the walls of the reactor and over-conversion in the axial element of the bed. This non-uniform conversion of hydrocarbon streams results in an ineificient utilization of the reactants and the formation of undesirable secondary products, such as heavy oils and tars, by excessive conversion of the reactant material.

In the first modification of the reactor wherein the pebbles are introduced axially, the reaction products are withdrawn through one or more effluent conduits intermediate the axis and the periphery of the reactor shall through the dome. In the second modification wherein the pebbles are introduced thru a plurality of conduits, the reaction products are removed through an axially positioned effluent conduit.

It is with the improvement in reaction conditions and attainment of more nearly uniform heating and reaction conditions in the reaction section of a pebble bed that the present invention is concerned. The principal object of the invention is to provide a process for the conversion of hydrocarbon material in vapor form which effects more nearly uniform conversion than has heretofore been possible and which minimizes undesirable side reactions in the conversion process. Another object is to provide an improved process for cracking vaporous hydrocarbon material. Another object is to provide an improved pebble heater type reactor. A further object is to reduce the slope and variation from the horizontal of the top surface of a gravitating pebble bed in a reaction zone. Other objects of the invention will become apparent from a consideration of the accompanying disclosure.

I have found that regulation of the flow rate of reactant gas through the pebble bed in a pebble heater reactor within a certain range can be utilized to decrease the angle of the conicalbed section, or sections, in the reaction portion of the pebble bed and thereby effect a flattening of the top surface of the pebble. bed'with concomitant improvement in the uniformity of heating and therefore reaction in the reaction section of the bed. It should be readilyapparent that any flattening of the top surface of. the bed in a pebble heater reactor improves the uniformity of the heating and reaction which takes place therein during conversion processes. -I have found that as gas flow rates are increased through the pebble bed the apparent weight of the pebbles is decreased to a point at which the apparent weight of the pebbles becomes zero and upon further increase the pebbles are lifted into the effluent gas stream and are carried out of the reactor in the gaseous eflluentline. Defining the maximumpermissible rate of gas flow through the pebble bed without lifting pebbles from the bed and effecting carry over of pebbles in the efiluent gas as G I have found that gas flow rates in the range of 0.5 G to G have a decided and material flattening effect on the top surface of the pebble bed in both embodiments of pebble induction referred to above. In the first embodiment wherein the top surface of the pebble bed is in the form of a single cone, gas flow rates in this range effect a reduction in the base angle of the cone or the angle of repose of the pebbles of more than 50%. In other words where the normal angle of repose of the pebble bed composed of a specific commercial type pebble is approximately 30, the angle of repose is reducedto approximately 13 at gas flow rates approaching G This decrease in the angle of repose of the pebbles in the top surface of the pebble bed greatly reduces the variation of the pebble bed from the horizontal and effects more nearly uniform gas flow, heating, and reaction than is obtained Without the high gas flow rates.

In the second embodiment of the pebble heater reactor discussed above wherein the hot pebbles are introduced in a multiplicity of streams into the upper section of the reactor gas flow rates in the range of 0.5 G to G effect a similar decrease in the angle of repose of the pebbles and proportionate flattening of the pebble bed as related to its original form.

At flow rates approaching G the upper section of the pebble bed may be regarded as in a stage of incipient fiuidization. The apparent weight of pebbles at the surface of a bed has reached zero and variations in flow of gas from the vertical have the effect of stacking pebbles directly vertically in line with each other and therefore cause them to readily roll toward the outer periphery of the bed. At the rate of flow designated G pebbles occasionally leave the pebble bed and return thereto without passing out with the effiuent. Incipient fluidization is not to be confused with complete fiuidization wherein the particles of heat-exchange material exist in dense phase in a fluidized or boiling condition in the reactor. It is believed that true fluidization of pebbles of uniform size cannot be attained, assuming that the term pebble is used in its usual connotation to denote generally spherical particles in the range of /5" to l" and greater in diameter.

A more complete understanding of the invention may be had from a consideration of the drawing in which Figure l is an elevation partly in section showing one embodiment of the invention; Figure 2 is an elevation partly in section of another embodiment of the reactor of the invention; and Figure 3 is a graph or curve showing the relationship G/G and the angle of repose of the top surface of a pebble bed.

Referring to Figure 1, 11 designates a pebble heating chamber connected by a throat 12 with a pebble reactor 13. A pebble outlet conduit 14 leads to a pebble chute 15 having a pebble feeder 16 therein connecting to the lower end of an elevator 17. The pebble conduit or chute 18 connects the upper end of elevator 17 with pebble inlet 19 and provides for re-cycling pebbles from the bottom of reactor 13 to the top of heater 11. Heater 11 is provided with conduit means 21 for introducing a hot combustion gas or a mixture of fuel and air into 4 the bottom of the heater, In the latter case, the fuel is burned in a combustion space or in the interstices of the pebble bed so as to heat the pebbles to a temperature above the desired reaction temperature. Conduit or stack 22 removes the combustion gas from the upper section of the heater.

Reactor 13 comprises an upper cylindrical section 23 and a lower cylindrical section 24 of substantially larger internal diameter so as to provide different pebble and gas mass flow rates in these two sections of the reactor. Throat 12 extends into the upper cylindrical section a substantial distance so as to provide a pebble free gas collector space 26 between the top of the pebble bed and the dome of the reactor. A line 27 provides for introducing an inert gas such as steam for minimizing carbon deposition in the upper section of the reactor. Product elfluent line 28 connects the vapor collecting space 26 with a pebble drop-out chamber 29 from which a return line 31 connected with conduit 15. Product line 30 is connected with the upper section of pebble drop-out chamber 29. Lines 32 provides for introduction of a blocking gas such as steam to prevent passage of product and/or heating gas into the elevator system and from chamber to chamber. Line 33 is a feed line for introducing a suitable feed to the bottom of the reactor in which there is a suitable gas distribution means 34. Line 36 in section 23 of reactor 13 indicates the top surface of a conical bed of pebbles in this section with the natural angle of repose. Dotted lines 37 and 38 indicate different locations of the top surface of the conical bed of pebbles under gas flow conditions between 0.5 G and max- Figure 2 illustrates a reactor of uniform horizontal cross section. Corresponding parts of this reactor are correspondingly numbered to similar parts of reactor 13 of Figure 1. The process of the invention is operative in the reactor of Figure 2 by controlling the flow rate of gas therethrough Within prescribed limits.

Various pebble heater controls and accessory equipment which are required in conventional operation of pebble heater equipment are aso applicable to the operation of the apparatus and process described but are not shown in the drawing.

While the invention is operable to advantage in the reactor of Figure 2 the reactor of Figure 1 provides much more variable operating conditions to suit requirements of specific processes. In general, section 23 of the reactor of Figure 1 encloses the reaction section of the reactor and expanded section 24 is primarily a preheaterfor the feed gas passing therethrough. The pebble volume of these two sections of the reactor may be correlated so as to confine substantially all of the reaction to section 23 where sharper heating results from the resulting higher gas velocity than is obtained in a reactor of uniform transverse cross section. As is well known in the art this is an advantage of considerable consequence in the cracking of normally gaseous and low boiling hydrocarbon material to ethylene and acetylene. Another advantage of the structure shown in Figure 1 lies in the lower pebble and gas mass flow rates provided in the expanded section 24 of the reactor.

Commercial alumina-mulite pebbles having an apparent density of 127ii-ft. in the form of A /s" spheres were introduced through an axial conduit to an 18" I. D. cylindrical reactor and a study was made of the angle of repose of the pebbles with increasing gas flow rates until incipient fluidization of the pebbles in the top of the bed was reached. The data obtained were plotted and a curve showing the relationship between G/G and the angle of repose of pebbles at the top surface of the bed is shown in Figure 3. A similar curve can be obtained for any type of pebble and will vary somewhat in accordance with the design of the reactor dome through which the effluent gas escapes from the gas collection space above the pebble bed and the characteristics of the pebbles.

In operating according to the invention it is practical to ascertain the G for any reactor design by increasing the gas flow rate through the reactor under operating conditions to the point at which pebbles occasionally are carried over in the efiluent gas. The flow rate at this point can be taken as G and operation can then be conducted with flow rates in the range of 0.5 to G to obtain the benefits and advantages of the invention in more nearly uniformly heating and reaction in the reaction section of the pebble bed in the reactor.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

I claim:

1. A process for pyrolytically converting hydrocarbon material to more desirable hydrocarbons which comprises gravitating at least one compact stream of hot refractory pebbles into the upper section of a reaction zone below the top thereof so as to maintain a pebblefree gas space above the hereinafter named bed and form a generally conical top surface for the pebble bed at the juncture of said stream with said bed said upper section being of substantially smaller diameter than a subjacent section whereby gas flow rate is materially faster in said upper section and pebble flow rate is materially slower in said subjacent section; gravitating said pebbles in a compact bed thru said reaction zone; recovering said pebbles from the lower section of said reaction zone; passing a hydrocarbon gas upwardly thru the bed of pebbles in direct heat exchange therewith so as to heat and convert said hydrocarbon gas to more desirable form; maintaining the flow rate of said hydrocarbon gas in the range of 0.5 G to G (where G is the maximum permissible rate of gas flow upwardly thru the pebble bed without lifting pebbles from the bed and efiecting carry over of pebbles in the effluent gas) so as to substantially reduce the base angle of each cone and thereby flatten the top surface of the pebble bed; and recovering a converted hydrocarbon gas from said pebblefree gas space.

2. The process of claim 1 in which the hydrocarbon conversion comprises cracking to lighter hydrocarbon material.

3. The process of claim 1 in which normally gaseous hydrocarbon is cracked to lighter hydrocarbons.

4. The process of claim 1 in which the pebbles are principally small spheres in the range of V8" to in diameter.

5. The process of claim 1 in which said pebbles are gravitated in a single stream into said zone axially thereof so as to form a single generally conical top surface on said bed of pebbles.

6. In a vapor-phase process for eifecting a chemical reaction in a pebble heater type reactor wherein a gravitating compact stream of refractory pebbles heated to a temperature above that of said reaction is introduced into a reactor thru a plurality of conduits extending into the upper section of said reactor so as to provide a pebble-free gas collecting zone in the upper end of said bed has an upright generally conical section directly below each pebble inlet of a slope corresponding to the natural angle of reposeof the pebbles; the improvement comprising passing a feed gas upwardly thru said pebble bed in direct heat exchange therewith at a flow rate in the range of 0.5 G to G (where G is the maximum permissible rate of gas flow upwardly thru the pebble bed without lifting pebbles from the bed and eflecting carry over of pebbles in the effluent gas) so as to substantially decrease the slope of each, said conical section and eifect a flattening of the top of said bed.

7. Apparatus comprising a closed pebble heater reactor having an upper cylindrical section of substantially smaller diameter than a subjacent cylindrical lower section wherein said upper section opens unrestrictedly into said lower section; gas inlet means in the lower portion of said subjacent section and gas outlet means in the upper portion of said upper section, said gas inlet and outlet means being the sole means for removing and introducing gas from and to, respectively, said reactor, whereby all of the gas introduced to said lower section thru said inlet means is removed from said upper section without loss, except by conversion to solid material, thru said outlet means; pebble outlet means in the bottom of said subjacent section; a single axially disposed pebble conduit extending into said upper section so as to deliver pebbles to a level below the top of said upper section, said pebble conduit being the sole means of introducing pebbles into said reactor.

References Cited in the file of this patent UNITED STATES PATENTS 1,984,380 Odell Dec. 18, 1934 2,448,257 Evans Aug. 31, 1948 2,519,340 Bailey Aug. 22 ,1950 2,548,030 Lefier Apr. 10, 1951 2,732,331 Wesh Aug. 2, 1951 2,585,984 Alexander et al. Feb. 19, 1952 2,623,842 Robinson Dec. 30, 1952 2,658,031 Schutte Nov. 3, 1953 2,684,929 Schutte July 27, 1954 2,692,903 Hachmuth Oct. 26, 1954 2,696,511 Bailey et al. Dec. 7, 1954 2,711,386 Delaplaine June 21, 1955 2,719,114 Letter Sept. 27, 1955 2,719,818 Findlay Oct. 4, 1955 2,760,851 Gilmore Aug. 28, 1956 2,766,073 Bergstrom Oct. 9, 1956 2,770,583 Haddad Nov. 13, 1956 OTHER REFERENCES vol. 44, page 201 

1. A PROCESS FOR PYROLYTICALLY CONVERTING HYDROCARBON MATERIAL TO MORE DESIRABLE HYDROCARBONS WHICH COMPRISES GRAVITATING AT LEAST ONE COMPACT STREAM OF HOT REFRACTORY PEBBLES INTO THE UPPER SECTION OF A REACTION ZONE BELOW THE TOP THEREOF SO AS TO MAINTAIN A PEBBLEFREE GAS SPACE ABOVE THE HEREINAFTER NAMED BED AND FORM A GENERALLY CONICAL TOP SURFACE FOR THE PEBBLE BED AT THE JUNCTURE OF SAID STREAM WITH BED SAID UPPER SECTION BEING OF SUBSTANTIALLY SMALLER DIAMETER THAN A SUBJACENT SECTION WHEREBY GAS FOLW RATE IS MATERIALLY FASTER IN SAID UPPER SECTION ADN PEBBLE FLOW RATE IS MATERIALLY SLOWER IN SAID SUBJACENT SECTION; GRAVITATING SAID PEBBLES IN A COMPACT BED THRU SADI REACTION ZONE; RECOVERING SAID PEBBLES FROM THE LOWER SECTION OF SAID REACTION ZONE; PASSING A HYDROCARBON GAS UPWARDLY THRU THE BED OF PEBBLES IN DIRECT HEAT EXCHANGE THERWITH SO AS TO HEAT AND CONVERT SAID HYDROCARBON GAS TO MORE DESIRABLE FORM; MAINTAINING THE FLOW RATE OF SAID HYDROCARBON GAS IN THE RANGE OF 0.5 GMAX TO GMAX (WHERE GMAX IS THE MAXIMUM PERMISSIBLE RATE OF GAS FLOW UPWARDLY THRU THE PEBBLE BED WITHOUT LIFTING PEBBLES FROM THE BED AND EFFECTING CARRY OVER OF PEBBLES IN THE EFFLUENT GAS) SO AS TO SUBSTANTIALLY REDUCE THE BASE ANGLE OF EACH CONE AND THERBY FLATTEN THE TOP SURFACE OF THE PEBBLE BED; AND RECOVERING A CONVERTED HYDROCARBON GAS FROM SAID PEBBLEFREE GAS SPACE.
 7. APPARATUS COMPRISING A CLOSED PEBBLE HEATER REACTOR HAVING AN UPPER CYLINDRICAL SECTION OF SUBSTANTIALLY SMALLER DIAMETER THAN A SUBJACENT CYLINDRICAL LOWER SECTION WHEREIN SAID UPPER SECTION OPENS UNRESTRICTEDLY INTO SAID LOWER SECTION; GAS INLET MEANS IN THE LOWER PORTION OF SAID SUBJACENT SECTION AND GAS OUTLET MEANS IN THE UPPER PORTION OF SAID UPPER SECTION, SADI GAS INLET AND OUTLET MEANS BEING THE SOLE MEANS FOR REMOVING AND INTRODUCING GAS FROM AND TO, RESPECTIVELY SAID REACTOR, WHEREBY ALL OF THE GAS INTRODUCED TO SADI LOWER SECTION THRU SAID INLET MEANS IS REMOVED FROM SAID UPPER SECTION WITHOUT LOSS, EXCEPT BY CONVERSION TO SOLID MATERIAL, THRU SAID OUTLET MEANS; PEBBLE OUTLET MEANS IN THE BOTTOM OF SAID SUBJACENT SECTION; A SINGLE AXIALLY DISPOSED PEBBLE CONDUIT EXTENDING INTO SAID UPPER SECTION SO AS TO DELIVER PEBBLES TO A LEVEL BELOW THE TOP OF SAID UPPER SECTION, SAID PEBBLE CONDUIT BEING THE SOLE MEANS OF INTRODUCING PEBBLES INTO SAID REACTOR. 